A PON is one type of optical access network for providing network access over “the last mile” of service. A PON is a point-to-multipoint network includes an optical line terminal (OLT) at a central office (CO), a plurality of optical network units (ONUs) at the user premises, and an optical distribution network (ODN) connecting the OLT and the ONUs. PONs may also include remote nodes (RNs) located between the OLTs and the ONUs, for example, at the end of a road where multiple users reside.
TWDM-PON stands for time and wavelength division multiplex passive optical networks. TWDM-PON is used for fiber to the home (FTTH) or FTTX, where X can be a premises, curb, or drop point, for example. In a TWDM-PON system, multiple wavelengths will be used for downstream and upstream transmission. For each wavelength, time division will be used; for multi-wavelengths, wavelength division will be used. There are Wavelength Division Multiplexers and Demultiplexers at the central office to combine downstream multi-wavelengths into a fiber and separate upstream multi-wavelengths into different receivers. At customer side, downstream wavelengths are transmitted to all tunable ONUs, and the tunable receiver of the ONU will pick up one wavelength from the downstream wavelengths. For upstream multi-wavelength light, which comes from tunable transmitters of the ONUs, the demultiplexer will separate the multi-wavelength light to different channel receivers at the OLT side.
Many PONs presently use wavelength division multiplexers to combine optical signals into a fiber for transmission. This approach introduces significant cost and complexity as new devices are added or deleted and the topography of the network changes. Also, this approach fails to provide automatic channel alignment and channel set-up for downstream and upstream channels at the system level.